Conventionally, liquid foods such as milk and other beverages have been sold while accommodated within, for example, packaging containers formed from a packing material having a paper substrate. Examples of such packaging containers include polyhedral packaging containers and brick-shaped packaging containers each having a flat top wall. Of these packaging containers, packaging containers having a capacity which enables a purchaser to drink liquid food in a single serving are formed through a process of longitudinally sealing a web-shaped packaging material into a tubular shape, transversely sealing and cutting the tube-shaped packaging material at predetermined intervals to thereby form initial-shape containers, and processing the initial-shape containers to complete the packaging containers. Further, a discharge opening is formed in the top wall of each packaging container and is covered with a pull tab from the outer side and with an inner seal from the inner side, and the pull tab and the inner seal are welded together.
During fabrication of the above described packaging container, heat is applied to the packaging material, the pull tab, and the inner seal in order to seal the packaging material in the longitudinal and transverse directions and to weld the pull tab and the inner seal together. At that time, stress acts on resin layers which constitute the pull tab and the inner seal, respectively, possibly resulting in formation of flaws such as pinholes and cracks in the packaging material. If such a flaw is generated in, for example, the innermost resin layer of the packaging material, an aluminum foil layer, and the inner seal, the liquid food contained in the packaging container soaks into the paper substrate, and oozes from an end surface of the packaging material or from the interface between the container body of the packaging container and the pull tab.
In view of the forgoing, there has been provided a flaw detection apparatus which samples packaging containers at proper intervals; forms an opening in the top wall of each sampled packaging container; charges water into the packaging container; and immerses a portion of the water-charged packaging container into water stored in a vessel. Since an end surface of the aluminum foil layer is exposed to the outside of the packaging container, when electrodes are dipped in the water within the packaging container and the water within the vessel, respectively, electrical continuity is established between the two electrodes if a pinhole has formed in the packaging container to a depth reaching the aluminum foil layer.
However, since the conventional flaw detection apparatus requires the labor of charging water into each packaging container and partially immersing the packaging container in water stored in the vessel, the labor for flaw detection is cumbersome. Further, the flaw detection apparatus cannot inspect all packaging containers. In addition, since the flaw detection apparatus forms an opening in the top wall of packaging containers to be inspected, the inspected packaging containers are destroyed.
An object of the present invention is to solve the problems involved in the above described conventional flaw detection apparatus and to provide a flaw detection apparatus which can simplify the labor necessary for flaw detection, which can inspect all packaging containers, and which does not destroy the packaging container during inspection.